Recently, negative differential resistance devices have attracted considerable attention due to their folded current–voltage characteristic, which presents multiple threshold voltage values. Because of this remarkable property, studies associated with the negative differential resistance devices have been explored for realizing multi-valued logic applications. Here we demonstrate a negative differential resistance device based on a phosphorene/rhenium disulfide (BP/ReS2) heterojunction that is formed by type-III broken-gap band alignment, showing high peak-to-valley current ratio values of 4.2 and 6.9 at room temperature and 180 K, respectively. Also, the carrier transport mechanism of the BP/ReS2 negative differential resistance device is investigated in detail by analysing the tunnelling and diffusion currents at various temperatures with the proposed analytic negative differential resistance device model. Finally, we demonstrate a ternary inverter as a multi-valued logic application. This study of a two-dimensional material heterojunction is a step forward toward future multi-valued logic device research.

promising and important 2D material. Graphene-based electronic and optoelectronic device applications [11][12][13][14][15] have been widely researched to take advantage of its many outstanding properties, such as high carrier mobility (10 000 cm 2 V −1 s −1 at room temperature), [16] excellent optical transparency (97% for monolayer), [17] high Young's modulus (0.5-1 TPa), [18] and wide absorption spectrum (300-1400 nm). [19] The unique properties of graphene have also triggered extensive research in other 2D materials including TMDs, particularly for various electronic [4,[11][12][13] and optoelectronic device applications. [3,5,14,15,24,31, 2D electronic devices have advanced from single junction to heterojunction devices and have recently been used in lateral/vertical field-effect transistors (FETs), [4,[11][12][13][62][63][64]67,68] negative differential resistance (NDR) devices, [42][43][44][45][46][47][48][49][50]66] and memory devices. [51][52][53][54][55][56][57][58][59][60][61]65] Especially, type-II heterojunctions have enabled efficient electron-hole separation during the conversion from light to current, and these have found use in high-performance 2D optoelectronic devices such as photodetectors, [3,5,14,15,24,31,[69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]102,103] photovoltaic devices, [84][85][86][87][88][89][90][91][92][93][94][95] and light-emitting diodes (LEDs). [87,88,[95][96][97][98][99][100][101] The electronic and optoelectronic properties of semiconducting TMDs depend on the number of layers due to quantum confinement effects. [104,105] For example, the transition from an indirect energy bandgap to a direct bandgap was observed for molybdenum-and tungstenbased TMDs as layer thickness decreases and approaches a monolayer. It has been reported that bulk molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ) have an indirect bandgap of 1.2 eV, whereas monolayer MoS 2 and WSe 2 have direct bandgaps of 1.88 eV and 1.65 eV, respectively, which lead to a high on/off-current ratio and high quantum efficiency. [106] Meanwhile, rhenium disulfide (ReS 2 ) exhibited a direct bandgap of 1.44 eV independent of its layer thickness, [107] and it is considered a promising candidate for future optoelectronic devices. [24,31,33,82,83] In this review, we first introduce important fabrication techniques of i) doping, ii) contact engineering, and iii) heterojunction formation for improving the performance of 2D-material-based devices in terms of fieldeffect mobility, on-current, responsivity, and temporal response (Section 2). We then discuss promising 2D-material-based electronic (Section 3) and optoelectronic (Section 4) devices,Since the rediscovery of graphene in 2004, this material has attracted an enormous amount of interest owing to its unique structural, mechanical, electronic, and optical properties. Beyond this, the unique properties of graphene have also triggered extensive research on other two-dimensional (2D) materials including transition metal dichalcogenides (TMDs), part...

Multivalued logic (MVL) devices/circuits have received considerable attention because the binary logic used in current Si complementary metal-oxide-semiconductor (CMOS) technology cannot handle the predicted information throughputs and energy demands of the future. To realize MVL, the conventional transistor platform needs to be redesigned to have two or more distinctive threshold voltages (Vs). Here, we report a finding: the photoinduced drain current in graphene/WSe heterojunction transistors unusually decreases with increasing gate voltage under illumination, which we refer to as the light-induced negative differential transconductance (L-NDT) phenomenon. We also prove that such L-NDT phenomenon in specific bias ranges originates from a variable potential barrier at a graphene/WSe junction due to a gate-controllable graphene electrode. This finding allows us to conceive graphene/WSe-based MVL logic circuits by using the I-V characteristics with two distinctive Vs. Based on this finding, we further demonstrate a light-triggered ternary inverter circuit with three stable logical states (ΔV of each state <0.05 V). Our study offers the pathway to substantialize MVL systems.

The effects of triphenylphosphine and (3-aminopropyl)triethoxysilane on a rhenium diselenide (ReSe2 ) photodetector are systematically studied by comparing with conventional MoS2 devices. This study demonstrates a very high performance ReSe2 photodetector with high photoresponsivity (1.18 × 10(6) A W(-1) ), fast photoswitching speed (rising/decaying time: 58/263 ms), and broad photodetection range (possible above 1064 nm).